The invention concerns a process for the manufacture of anthraquinone through the oxidation of o-benzyl-toluene with air or oxygen-containing gases in an acetic acid solution in the presence of cobalt, manganese and bromide salts at temperatures of about 100.degree.-150.degree. C. and atmospheric or elevated pressures.
Anthraquinone is a starting material in the manufacture of valuable dyes which are required by the textile industry in ever-increasing amounts.
From DT-OS 20 50 799, it is known to oxidize o-benzyl-toluene with air in acetic acid solution with the addition of catalytic amounts of cobalt bromide and manganese acetate at a temperature of 160.degree. C. and a pressure of 20 bar. The weight ratio of o-benzyl-toluene to acetic acid approaches about 1:12. The o-benzoyl-benzoic acid is not isolated, but instead is directly converted to a raw anthraquinone under heating with concentrated sulfuric acid; purification is effected by extraction with benzene. An unpurified anthraquinone crystallizes out with a melting range of 279.degree.-282.degree. C. in a yield of 72% of theory, calculated from the starting o-benzyl-toluene.
It is further known from the Japanese patent publication 49-75563 (Ref: Chem. Abstr. 1975, 43096w) to oxidize o-benzyl-toluene in an aliphatic carboxylic acid, for example acetic acid, in the presence of cobalt salts and bromine compounds, to which manganese salts may also be added as additional catalytic agents. The reaction, carried out at the atmospheric pressure or slightly elevated pressure at temperatures from 70.degree. to 200.degree. C. with air or oxygen-containing gases, yields o-benzoyl-benzoic acid with an anthraquinone content of between 3 and 9% as product. The oxidation in the acetic acid solution is carried out in general with a weight ratio of o-benzyl-toluene to acetic acid of about 1:10. After a reaction time of 4-5.5 hours o-benzoyl-benzoic acid in a yield of about 90-92.5% of theory is recovered, along with about 3-4% of theory of anthraquinone. The purity of the o-benzoyl-benzoic acid is not further discussed. The product is then heated with a minimal amount of concentrated sulfuric acid to about 250.degree.-350.degree. C.; in order to purify the thus-obtained anthraquinone, it is distilled at normal pressure. The degree of purity of this product is also not given. It is further noted that it is advantageous to add to the reaction mixture a solvent which does not mix with water such as benzene, chlorobenzene or dichlorobenzene in order to eliminate the water produced by the reaction as an azeotrope. For example, half of the acetic acid can be replaced according to the publication by chlorobenzene; however, an azeotropic distillation during the oxidation is not achieved by following the example of the Japanese publication.